The present invention relates to semiconductor processing and more specifically to a method and apparatus for detecting the onset of planarization during chemical mechanical planarization operations.
A challenge for chemical mechanical planarization processes, especially with respect to copper interconnects, is to remove the copper overburden without causing excessive dishing of copper in the trenches. Similarly, a typical requirement is to have minimal erosion and oxide loss in areas that cannot withstand aggressive chemical mechanical planarization (CMP) due to the density of fine copper trenches and vias in the underlying oxide or low-K dielectric layer. FIGS. 1A and 1B are exemplary schematic diagrams illustrating erosion and dishing, respectively. Dishing occurs when a top surface of a copper trench/via line 104 recedes below or protrudes above the level of adjacent dielectric 102, as illustrated in FIG. 1B. Erosion is a localized thinning of dielectric 102 as illustrated in FIG. 1A. Erosion of the dielectric occurs from over polishing, which is generally necessary to make sure that all the metal is removed from the top of the dielectric (otherwise lines may be shorted together). Dishing occurs on a soft metal, e.g., copper. Through dishing, the thickness of the copper interconnects 104 may be reduced which increases the electrical resistance. In addition, the overall changes in the planarity caused by dishing and erosion can also lead to challenges in obtaining good focus across the die during subsequent lithographic steps. However, for typical CMP processes, the approach generally taken is a one-size fits all approach. That is, the process to achieve a planarized copper top surface and the process from the planarized copper top surface to the clearance of the copper are treated under the same hardware, consumable and process conditions. One shortcoming with this approach is that the conditions for planarizing the copper overburden and the conditions for clearing a planarized layer are different. Thus, a single set of processing parameters, or consumables, may not be optimal for both of these stages of the CMP process.
In view of the foregoing, there is a need for a method and apparatus to provide a reliable signal indicating a point where processing conditions may be adjusted at or sometime after the onset of planarization.